The present invention relates to stabilized antenna platforms for ships and other floatable vessels and in particular to stabilized platforms for directive ship mounted antennas for satellite communication to compensate for roll and pitch movements of the ship. The invention further relates to platforms comprising an antenna dish with elevation and azimuth control means, as well as counterweight means, supported in a universal joint bearing.
Typically a ship may be subject to pitch and roll movements in the order of .+-. 6.degree. - 10.degree. and .+-. 20.degree. - 30.degree. respectively, dependent on ship type and loading conditions. It has been recommended by experts that a satellite communication ship terminal should have a G/T = -10dB. G/T denotes the ratio between antenna gain and the noise temperature of the receiving system. This corresponds to a net antenna gain of approximately 18 dB.
An antenna with a net gain (at the -3 dB points) of 18 dB will have a beamwidth of about 15.degree.. It is thus obvious that even though the antenna must be pointed in the direction of the satellite, the accuracy of the pointing mechanism needs not be very high.
Antenna platforms are usually stabilized by means of gyro mechanisms, but such stabilizing systems are rather expensive and give higher accuracy than is now considered necessary.
In a paper by Mr. R. J. Kirkby, "A single stabilized antenna platform for maritime satellite communications" in Conference Publication No. 95 "Satellite Systems for Mobile Communications and Surveillance" published in 1973 by The Institution of Electrical Engineers, England, there is described a simple pendulum stabilized antenna platform.
The platform suggested by Mr. Kirkby is constructed as part of a pendulum with low natural frequency. The platform will then be relatively insensitive to ship movements at frequencies significantly higher than its natural frequency. Thus by keeping this natural frequency sufficiently low, the influence of the most severe ship movements can be reduced. It is stated that pointing accuracies better than .+-. 5.degree. can theoretically be achieved in beam-sea conditions corresponding to Beaufort 9 winds. The accuracy is dependent on ship type.
One factor which limits the accuracy that can be obtained with this known platform is that the necessary damping of its movements is dependent on friction in the platform gimbal bearings. The amount of friction to be used must then be a compromise between the desired degree of damping and the amount of coupling that can be tolerated.